In DEP (Direct Electrostatic Printing) the toner or developing material is deposited directly in an image-wise way on a receiving substrate, the latter not bearing any image-wise latent electrostatic image. The substrate can be an intermediate endless flexible belt (e.g. aluminium, polyimide etc.). In that case the image-wise deposited toner must be transferred onto another final substrate. Preferentially the toner is deposited directly on the final receiving substrate, thus offering a possibility to create directly the image on the final receiving substrate, e.g. plain paper, transparency, etc. This deposition step is followed by a final fusing step.
This makes the method different from classical electrography, in which a latent electrostatic image on a charge retentive surface is developed by a suitable material to make the latent image visible. Further on, either the powder image is fused directly to said charge retentive surface, which then results in a direct electrographic print, or the powder image is subsequently transferred to the final substrate and then fused to that medium. The latter process results in an indirect electrographic print. The final substrate may be a transparent medium, opaque polymeric film, paper, etc.
DEP is also markedly different from electrophotography in which an additional step and additional member is introduced to create the latent electrostatic image. More specifically, a photoconductor is used and a charging/exposure cycle is necessary.
A DEP device is disclosed in e.g. U.S. Pat. No. 3,689,935 This document discloses an electrostatic line printer having a multi-layered particle modulator or printhead structure comprising:
a layer of insulating material, called isolation layer; PA1 a shield electrode consisting of a continuous layer of conductive material on one side of the isolation layer; PA1 a plurality of control electrodes formed by a segmented layer of conductive material on the other side of the isolation layer; and PA1 at least one row of apertures. PA1 a means for delivering charged toner particles, said means having a toner bearing surface coupled to a means for applying a first electric potential to said surface, PA1 a means for creating a flow of said charged toner particles away from said surface, PA1 a means for passing an image receiving substrate in said flow, PA1 a printhead structure having printing apertures and control electrodes, placed between said toner bearing surface and said image receiving substrate, leaving a gap (d) between said toner bearing surface and said control electrodes, characterised in that PA1 said control electrodes are coupled to a means for generating a first AC-field on said control electrodes and that PA1 a means for selectively switching said first AC-field on and off in accordance with image data is placed between said control electrodes and said means for generating said first AC-field for image-wise controlling said flow of toner particles. PA1 providing charged toner particles on a surface of a means for delivering toner particles, PA1 creating an electric potential difference between said surface and an image receiving substrate for creating a flow of charged toner particles towards said image receiving substrate from surface, PA1 interposing a printhead structure, with printing apertures and control electrodes in said flow of toner particles, for image-wise controlling said flow of toner particles, PA1 selectively switching an AC-field on and off, in accordance with image data, between said control electrodes and said toner bearing surface, PA1 depositing said image-wise controlled flow of toner particles on said image receiving substrate and PA1 fixing said toner particles to said substrate.
Each control electrode is formed around one aperture and is isolated from each other control electrode.
Selected electric potentials (only DC-potentials) are applied to each of the control electrodes while a fixed potential is applied to the shield electrode. An overall applied propulsion field between a toner delivery means and a support for a toner receiving substrate projects charged toner particles through a row of apertures of the printhead structure. The intensity of the particle stream is modulated according to the pattern of potentials applied to the control electrodes. The modulated stream of charged particles impinges upon a receiving substrate, interposed in the modulated particle stream. The receiving substrate is transported in a direction orthogonal to the printhead structure, to provide a line-by-line scan printing. The shield electrode may face the toner delivery means and the control electrodes may face the receiving substrate. A DC-field is applied between the printhead structure and a single back electrode on the receiving substrate. This propulsion field is responsible for the attraction of toner to the receiving substrate that is placed between the printhead structure and the back electrode.
A DEP printer wherein the printhead structure is a mesh instead of a insulating base with printing apertures trough this base has been disclosed in U.S. Pat. No. 5,036,341. In this disclosure it is taught to introduce an AC-field with frequency between 2 and 5 kHz and peak voltages between 500 and 2000 V on the toner delivery means in order to speed up the printing.
One of the recognised problems with both of these types of printhead structures, the printing apertures are easily clogged by toner particles when the DEP device is used over a longer period of time.
This problem of clogging of the printing apertures has been addressed in several ways. In e.g. U.S. Pat. No. 4,491,855 different measures are disclosed to minimise clogging. It is proposed in this disclosure to provide a conveying member on which a layer of toner particles is deposited and to apply an AC voltage (300 V peak to peak and frequency of 4.5 kHz) between the toner conveying member and the continuous layer of conductive material (shield electrode) on the printhead structure. Due to this AC voltage the toner particles "jump" between the toner conveying member and the surface of the printhead facing said toner conveying member. It is believed that the "touching" toner particles will assist in delaying the contamination of the printhead structure and clogging of the apertures. In this disclosure also a special design of the apertures in the printhead structure and a special selection of the material from which the printhead structure is made is also claimed to assist in delaying the clogging. A last measure which is proposed is to `clean` the printhead structure by periodical electric bursts (spark discharges). Also in U.S. Pat. No. 4,478,510 the use of a spark discharge to remove toner particles adhered to the printhead is disclosed.
In U.S. Pat. No. 4,876,561 clogging of the printhead is prevented by making the apertures large enough and/or the thickness of the isolating layer small enough.
In U.S. Pat. No. 4,903,050 an AC voltage is applied to the back electrode as in U.S. Pat. No. 4,755,837, but this disclosure recommends the addition of a shutter and vacuum system is in order to prevent the dislodged toner to fall onto the receiving substrate.
In U.S. Pat. No. 5,095,322 clogging of the apertures is prevented by applying to the shield electrode a pulsed DC-voltage which is 180.degree. out of phase if compared with the AC-voltage applied to the charged toner conveyor. In an other embodiment a DC-biased AC voltage is applied to the shield electrode with the same frequency as the AC voltage applied to the charged toner conveyor but 180.degree. out of phase is used to prevent clogging of the apertures in the printhead.
Also mechanical ways to prevent clogging or to clean the printing apertures have been disclosed. In, e.g., U.S. Pat. No. 5,153,611, U.S. Pat. No. 5,202,704, U.S. Pat. No. 5,233,392 it is disclosed to prevent clogging of the printing apertures by using an ultrasonic vibration applied to the printhead. In U.S. Pat. No. 5,283,594 the level of vibration applied to the printhead is different during writing time and cleaning time. In U.S. Pat. No. 5,293,181 the printhead is vibrated in such a way that a mechanical propagating wave is created.
In U.S. Pat. No. 5,307,092 an anti-static coating is applied to the electrodes in the printhead so that any tribocharge that accumulates during writing can be grounded. As a result the net tribocharge on the printhead (which is unwanted and is responsible for unpredictable results and clogging) is removed and a better long-time performance results.
In WO-A-90 14959 the printhead is treated with pressurised air or vacuum so that the individual toner particles do not adhere to the printhead for such a large amount if compared with a printing engine not using the air treatment. In the same document an additional improvement is described where by the magnetic toner particles are removed from the printhead by using a much stronger magnetic field during the cleaning cycle than during the writing cycle.
In U.S. Pat. No. 4,755,837 an AC voltage is used for the backing electrode during the cleaning cycle. In a preferred embodiment the AC voltage on the back electrode is phase shifted by 180.degree. if compared with the AC field (400 V peak to peak, no frequency disclosed) that is used upon the charged toner conveyor which is needed to obtain a high toner mist production, leading to high optical densities and short printing times. Further on the AC voltage can also have a certain DC-offset.
In U.S. Pat. No. 5,526,029 it is disclosed to use ionised air for blowing over the printhead so that the electrostatic interaction of the toner particles with the printhead is reduced and the toner particles are removed more easily from it than if compared with patent application WO-A 90 14959 where the air used is not pre-treated at all.
In EP-A-780 740 a printhead structure, for a DEP (Direct Electrostatic Printing) device is disclosed that comprises an insulating material, a slit, formed by two sides (SA and SB) of said insulating material, as printing apertures and control electrodes characterised in that only one of said two sides forming said slit carries control electrodes. In such a printhead structure the chance of clogging of the printing apertures is lower than in printhead structures wherein fine (maximum dimension around 400 .mu.m) circular, elliptical, rectangular or square printing apertures are used.
In U.S. Pat. No. 5,625,392 an edge electrode is described so that instead of individual apertures or a larger slit as described in EP-A-780 740 an even larger free zone between the toner applicator and the receiver exists, resulting in even better properties regarding clogging of the printhead structure.
Said edge electrode system proposed in U.S. Pat. No. 5,625,392 suffers however from the drawback that, in order to obtain a good image contrast between image parts of low density and image parts of high density, the overall applied propulsion field between the toner applicator and the receiver on the back electrode must be set to a rather low value, leading to only a moderate printing speed.
In U.S. Pat. No. 5,374,949 an AC-field is superimposed upon the voltage applied to the individual control electrodes. Two different implementations have been described. In the first one image density is obtained if an AC-field is set between the toner delivery means and the back electrode while the control electrodes are kept at the ON-voltage. An additional AC-voltage can be applied to said control electrodes. In the second implementation said AC-voltage is applied to the control electrodes in the OFF-state. So it is described in said patent application that image density is regulated by switching over from a first DC-voltage to a second DC-voltage for said control electrodes, while on one of said DC-voltages an additional AC-voltage is superimposed.
Thus there is still a need for further improved DEP devices with enhanced printing speed and less clogging that are stable in time.